Field of the Invention
The present invention relates to a bituminous composition in the form of granules comprising a core and a coating. The invention relates also to a method for preparing said composition.
Bituminous compositions comprising a binder matrix are nowadays commonly used for producing anhydrous binders. These compositions are also used in emulsion form for road applications. In order to improve the physicochemical characteristics, the bituminous compositions comprising a binder matrix may especially coat polymers dispersed therein.
Generally, the preparation of such compositions is lengthy and is carried out in dedicated plants having mixing tanks arranged to receive the various components and additives forming the composition. The tanks can comprise agitating, grinding, dispersing and shearing tools or other elements which contribute to the maturation of the composition. Extruder-type devices are used and have the advantage of permitting a continuous preparation process in a short time. However, the use of extruders is sometimes associated with disadvantages, as will be seen below.
Depending on the desired composition, the preparation tools can include systems permitting the addition of crosslinker. The conditions in which crosslinking or dispersion takes place, where appropriate, can vary depending on the final composition that is desired. This can affect the rates of production and/or the preparation costs.
In addition, the use of certain additives can have a marked environmental impact. Moreover, the raw materials used and their physicochemical quality have a considerable impact on the physicochemical properties (including the thermomechanical performances) of the final composition. There is a constant need to monitor and standardize the raw materials that are used. Such monitoring is a lengthy and expensive process.
During the method for preparing the compositions, the quantities of energy are very considerable, in particular as regards the heating of kneading tanks, which can sometimes contain a large quantity of components. Heating as such must also be monitored so that the thermomechanical performances of the final composition are not impaired.
It is also very important for the bituminous compositions to have good storage characteristics (especially homogeneity).
Description of Related Art
Document FR 2 871804 proposes a method for preparing a mixture comprising bitumen and at least one polymer in an extruder at variable temperature.
It has been shown that the particle size of the polymer plays a vital role in the preparation of bituminous compositions. The finer the particle size of the polymer, the higher the dispersion rate thereof in the bitumen. However, the cost of conditioning the polymers in powder form is all the higher, the smaller the size of the particles of said powder. Moreover, such conditioning presents certain risks and requires that expensive safety measures be put in place.
In addition, attempts have been made to mix the polymer in liquid form with the binder matrix of the bituminous composition, which results in a very short kneading time and an almost instantaneous mixture. However, the energy necessary for liquefying the polymer is very considerable and the time required is long. Also, this phase is carried out at temperatures which can cause degradation of the polymer.
Generally, the use of extruders for preparing binder matrix compositions requires very precise operating conditions. There is therefore a constant need to improve the preparation methods in order to remedy the disadvantages mentioned above. Very particularly, there is a need to limit the costs, to have freedom of choice over the starting material and especially to provide a composition. Stability over time is understood as meaning especially the possibility of storage for prolonged periods and at ambient temperature.
The present invention will improve the situation.